I think a new extension is the way to go. We could probably call it
GL_MESA_depth_clip_disable or GL_MESA_depth_clamp_d3d.

One could try to combine the CLIP_DISABLE register with clamping gl_FragDepth
inside the fragment shader. I'd expect the cost of this to be way too high,
even if it's just enabled when both GL_DEPTH_TEST and GL_DEPTH_CLAMP are
enabled. Maybe some considerations similar to GL_ARB_conservative_depth can be
applied, but I guess that'd require hardware support.